The invention pertains to a high-current rotary connection for the leadthrough of electric power lines to movable electrical loads in closed spaces.
Rotary high-current connections of the type cited above are required to supply large operating currents to closed spaces, when, for example, limited rotational and pivoting motions must be executed between structural components inside and outside the boundary walls of the spaces. These requirements are present in the case of, for example, systems such as melting and casting units supplied with electric current, especially with medium-frequency alternating current, in which the molten material is poured out by tipping a crucible, the crucible forming a single structural unit with the heating device. High-current connections of the type described above, however, are not limited to uses only in melting and casting furnaces.
It is important that the high-current connections of the general type in question are also intended to serve simultaneously to supply and carry away coolants, by means of which furnace components such as the induction coils used in induction melting processes can be protected from overheating.
In systems where there are pressure differences between the two sides of the boundary walls of the closed spaces, such as in the case of vacuum furnaces, for example, there are also special requirements on the leak-tightness of the high-current connections.
High-current connections of the type described above are known (corresponding to U.S. Pat. Nos. 4,492,423 and 5,127,836). from, for example, DE 41 22 574 A1 and DE 32 19 721. The high-current connection described in DE 41 22 574 A1 consists essentially of a stationary, coaxial arrangement of an electrically conducting internal pipe and an external pipe together with a rotatable current conductor arrangement which is coaxial to the stationary coax conductor unit. By means of flexible current bridges, the first coax conductor unit is electrically connected in a bipolar manner to the second coax conductor unit. The coax conductor units known from DE 41 22 574 A1 open at the ends into four adjacent, ring-shaped metal flanges, which are arranged concentrically to each other in pairs and are connected to each other by stranded wires bent into the shape of U's in such a way that in each case the inner and outer ring-shaped flanges are at the same potential. The inner, rotatable ring flanges have essentially the same diameter and close off on the one side the end of the internal coax pipe and on the other side the end of the outer coax pipe. The stranded wire bundles of the one potential are approximately mirror images of the stranded wire bundles of the other potential. Care is taken to ensure that the pivot angle is sufficiently large by making the loops of the stranded wires sufficiently long.
The electric current is supplied to each of the outer, stationary flanges by means of a radially oriented current conductor.
In a design of the type described above, e.g., that according to DE 32 19 721, the problems associated with thle difficulty of transferring high electrical currents efficiently, especially in the upper medium-frequency range, have been found to be disadvantageous. Thus, for example, the electric current is introduced to the opposing flange in only a local manner, that is, only within the confines of the part of a sector situated at the outer radial edge. As a result, the electric current being supplied is not distributed uniformly or completely over the ring flanges. The solution proposed in DE 41 22 574 A1, namely, to provide the current-supplying coax conductor arrangement with a certain minimum length, does not solve this problem completely, even though that is the goal. Because the electric current always tries to flow along the shortest possible route, the current is not distributed homogeneously around the opposing ring-shaped flanges. Instead, the disadvantageous situation develops that the current flows into the opposing flange almost exclusively through the stranded wires near the external coax feed lines adjacent to the current feed conductor. This leads to differences in the thermal loads on the stranded wires, which is disadvantageous; as a result, these stranded wires are sometimes produced with different diameters, even though this increases the production cost.
The coaxial current conductor arrangement known from DE 41 22 574 A1 also suffers from the disadvantage that the alternating current resistance of the current-carrying coax conductors and the stranded wire conductors increases as a result of the increasing resistance at higher operating frequencies. Higher resistances, however, bring with them the disadvantage of extra thermal loads on the individual stranded wire conductors. Although, when new and still protected by a sound layer of insulation, these wires can handle high current loads, the continuous rotational movement and the associated abrasion of the stranded wires nevertheless leads to a current distribution in the stranded wires similar to that found in uninsulated stranded wire conductors. Such uninsulated stranded wire conductors, however, are unsuitable for high alternating currents at high operating frequencies, because they are associated in a disadvantageous manner with power transfer losses.